The interest over photosynthetic microbes such as microalgae and cyanobacteria as a source of bioproducts has expanded significantly in the last years. This interest has been promoted by the price fluctuations and security risks associated with fossil fuels. The need to reduce costs of fuels has promoted the interest in other microalgal bioproducts that can be even more attractive economically than the biofuels. These products can include pharmaceuticals, nutraceuticals, fertilizers, feeds, food additives, and pigments among others.
Although the microalgal productivity is higher than that of other crops, with up to 2,470-12,345 gal oil (hectare·yr)−1, there are some limitations that need to be addressed to make it competitive. One of the major bottlenecks in the use of biofuels and other bioproducts is the availability of light[1-3]. In cultures maintained outdoors, that use available natural light, the depth and hence the productivity of the culture is limited by the light penetration and availability in a given area. For cultures maintained with artificial light, the energy for illumination can be a significant part of the cost of microalgal biomass production, reaching in some cases 50% or more of the costs[4-7]. Besides the light quantity, the wavelength distribution also affects the microalgal biomass production. It has been found that the wavelength affects not only the biomass productivity, but also its composition.
Several strategies have been explored to reduce the cost of light and increase the areal productivity of the microalgae. Among these strategies are thin layer photobioreactors, stacked reactors with waveguides, tubular photobioreactors, submerged light strips and many others. None of these methods overcome the basic lighting problems: 1) for cultures using natural light, the fixed limited availability of light energy per unit of area, the diurnal cycle (night/day) and the depth of light penetration; and 2) for cultures with artificial light, the cost of the electrical energy to provide illumination and the light penetration.